The phylum embraces heterotrophic species with a heterokaryotic nuclear apparatus, multiple cilia over body, pellicular alveoli, and always an infraciliature (highly developed cortical system with kinetid as central feature). Ciliates play a significant role in food chains and nutrient recycling; most taxa are found free-living in marine or freshwater habitats, while about one third are symbiotic forms, less then 10 percent are possibly endemic in soils or known only as fossils, respectively.